I'm going over some boson practice tests, and I was surprised to see that the process ID specified as part of "router ospf" the command. I'd appreciate it if someone could tell me why a process ID is required. Would a single router have multiple instances of the OSPF process?
You got it. Process IDs are used because a single router could run multiple instances of OSPF. Remember, though, in OSPF the process IDs don't have to match on two routers for them to become neighbors: They are only locally significant. EIGRP AS numbers DO have to match.
According to Cisco's OSPF Design Guide even though is possible to run multiple OSPF processes on the same router, is not recommended as it creates multiple database instances that add extra overhead to the router. I personally don't know of any specific example of when running multiple processes would be required. However, I don't think that's a concern for a CCNA level candidate.
Anyhow, here's another thread with some good info as well: https://learningnetwork.cisco.com/thread/6248
The only real life example would be if you are overhaulin your network. I had a similar experience where we had to change a whole bunch of stuff on EIGRP in order to meet customer needs while the network was live. So you could theoretically run a second process with a lower administrative distance, change all kinds of stuff within your routing while your production environment is still up and running. Once you are ready for implementation, lower the administrative distance of the old process and make the bring the new process back to default. Once you validate, delete the old process. I don't think people run into this every day, but the option is out there.
You can create multiple ospf processes on the same router. Each ospf process has its own routing data.In addition, routing data is not shared between ospf processes automatically. To do this, you should use the redistribute command.
hope this help,